The primary objective of this protocol is to evaluate the tolerability of cinacalcet in individuals with fibroblast growth factor 23 (FGF23)-mediated hypophosphatemia, using an open-label, dose-titration study of once-daily dosing. Secondary objectives are to evaluate the pharmacodynamics of cinacalcet in this subject population and to explore the efficacy of cinacalcet by comparing a) level of oral phosphate required at baseline to the level required at maximum tolerated dose (MTD) and b) change in renal phosphate handling from baseline to MTD. Tertiary objectives are to evaluate tolerability, pharmacodynamics, and efficacy of twice daily dosing of each subject s MTD of cinacalcet after completion of the once-daily dose-titration phase. A final objective is to determine the length of time it takes for subjects to return to their pre-treatment steady state once treatment is complete.